Calculate distance between two points

So whilst working with Google Maps lately, I found this super-useful snippet that calculates the distance between two points, using longitdue and latitude.

Code:

function distance($lat1, $lon1, $lat2, $lon2, $unit) { 
$theta = $lon1 - $lon2;
$dist = sin(deg2rad($lat1)) * sin(deg2rad($lat2)) + cos(deg2rad($lat1)) * cos(deg2rad($lat2)) * cos(deg2rad($theta));
$dist = acos($dist);
$dist = rad2deg($dist);
$miles = $dist * 60 * 1.1515;
$unit = strtoupper($unit);

if ($unit == "K") {
return ($miles * 1.609344);
} else if ($unit == "N") {
return ($miles * 0.8684);
} else {
return $miles;
}
}

Usage:

echo distance(32.9697, -96.80322, 29.46786, -98.53506, "k")." kilometers";

Credit: PHPSnippets.info